In the manufacture of pressure-sensitive recording paper, better known as carbonless copy paper, a layer of pressure-rupturable microcapsules containing a solution of colorless dye precursor is normally coated on the back side of the top sheet of paper of a carbonless copy paper set. This coated backside is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dye by reaction with the dye precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dye precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dye precursor, then reacts with the color developer in the areas in which the dye containing microcapsules were ruptured, thereby effecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording papers is well known.
Among the well known basic, reactive, colorless chromogenic dye precursors used for developing colored marks when applied to a receiving sheet are such color developers are Crystal Violet Lactone (CVL), the p-toluenesulfonate salt of Michler's Hydrol or 4,4'-bis(diethylamino) benzhydrol, Benzoyl Leuco Methylene Blue (BLMB), Indolyl Red, Malachite Green Lactone, 8'-methoxybenzoindoline spriopyran, Rhodamine Lactone, and mixtures thereof.
Among well known color developers used on CF sheets are activated clays, zinc salicylate, and phenolic-type resins, such as acetylated phenolic resins, salicylic acid modified phenolics and, particularly, novolac type phenolic resins.
Traditionally CF coatings have been applied to a support sheet such as a paper substrate via a coating station on a paper machine or on an off-line coater. This has been true for most of the different types of CF coatings in use today, whether it involves activated clays, zinc salicylate, the phenolic resins or combinations thereof. In each of the above cases, printers must buy and inventory several different basis weights and colors of each: CB, CFB, and CF. This is true for both the sheet-fed printers as well as the continuous (roll) printers. There are also commercially available CF coatings which are dissolved in a suitable solvent or ink system. These can be applied to the substrate on a printing press by a variety of methods among which are flexographic, lithographic or transfer letter press. These CF printing inks partially eliminate the inventory problem by enabling the printer to inventory only CB and uncoated paper of the various colors and weights.
In terms of the CF sheet various CF coatings and formulations have been used and various methods of applying the CF coating or formulation have been tried. According to the prior art such coating was carried out with an aqueous coating composition over the entire surface of the substrate, such as an aqueous slurry of finely ground phenolic resin, as shown in U.S. Pat. No. 3,672,935 and numerous other patents. The process described in these patents has the disadvantage that, following application of the coating composition, the water must be evaporated and this requires considerable energy. Additionally, the need for drying requires the use of a complex and expensive apparatus for an aqueous coating composition.
It is also known that acidic color developer, such as phenolic resin, can be applied to the paper substrate as a solution in a volatile organic solvent which after application evaporates completely leaving a thin film of solid resin on the paper. The latter method is taught in U.S. Pat. Nos. 3,466,184 and 3,466,185. The consistency of these solid solutions and the necessarily volatile solvents used therein give rise to printing problems and cause swelling of rubber plates and rolls. Furthermore they have high energy requirements and potential environmental contamination problems.
In addition to possible problems with volatile organic emmisions, organic solvent-based CF coatings have a tendency to transparentize the substrate. Accordingly, in U.S. Pat. No. 4,337,968, assigned to the same assignee as the present invention, it is suggested that non-volatile organic solvents be used. It is stated in that patent that in combination with the phenolic resin, a fatty alcohol, and an amorphous lipophilic silica, the non-volatile solvent improves the characteristics of the coated film because there will be a residue of solvent in the solidified gel structure of the spongy color developer film thereby produced. This high boiling point solvent, as retained in the deposited film, tends to act as a plasticizer and give flexibility to the deposited film, preventing excess dusting during subsequent printing and handling. Having the solvent in small quantities in the deposited film acts in a way to attract the lipophilic components normally found in conventional CB coated systems. Thus, at the instant of impact, the liquid organophilic nature of the CB dye system is more readily attracted into the slightly wetted highly organophilic surface. While the system of U.S. Pat. No. 4,337,968 is a distinct improvement over other organic solvent-based CF coatings, it may still be more desirable to use an aqueous-based CF coating in order to avoid image decline and spread due to aging if the solids content of the ink is high enough to avoid the problems found with typical aqueous-based CF coatings.
The major problem with aqueous CF printing coatings is the large drying capacity required. Traditional water based CF coatings range 4 to 8 gm/m2 dry and are applied from a 25 to 50% solids coatings. A 4 gm/m2 coating at 50% solids requires the removal of 4 grams of water per square meter. A 6 gm/m2 coating at 25% solids requires the removal of 18 grams of water per square meter, i.e., ##EQU1##
In addition, when this amount water is added to a paper substrate, numerous controls must be added to the coater to prevent sheet distortion, curl or cockle. Special grades of paper are also required to avoid excessive penetration and web breaks on the coater. The result is a coater and accompanying facilities that are very expensive to build and operate. The centralization of production, due to high capitalization costs, also produces cost inefficiencies in the form of high scrap levels and transporation costs. All of these factors add to the cost of the resulting CF sheet.
Accordingly, the need remains for high solids content, aqueous, CF printing inks which contain a color developer and for carbonless copy paper coated with such a high solids CF printing ink.